Lithographic apparatus, illumination system, illumination controller and control method

ABSTRACT

A lithographic apparatus includes an illumination system configured to condition a radiation beam. The illumination system includes a pulsed source of radiation and a controller to control an output of the pulsed source of radiation. The controller includes a dose sensor to measure a dose of a pulse of the source of radiation. The dose sensor includes a dose sensor output to provide a dose signal representative of the measured dose. An integrator unit is connected to the dose sensor output. The integrator unit integrates the dose signal at least twice, an output of the integrator unit provides an integrator output signal including the at least twice integrated dose signal. The output of the integrator unit drives a driving input of the source of radiation with the integrator output signal.

BACKGROUND

1. Field of the Invention

The present invention relates to a lithographic apparatus, anillumination system, a controller and method to control an output of apulsed source of radiation.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In a lithographic apparatus, as well as in many other applications, asurface such as a substrate surface or a wafer surface is to beilluminated by a pulsed source of radiation. The pulsed source ofradiation, such as a pulsed laser, provides for a series of pulses,e.g., at a certain pulse repetition rate. During an illumination, thesubstrate or other object, which is to be illuminated, is moved suchthat with each pulse a different part of the surface of the substrate orother object is illuminated. Commonly, surfaces which are illuminated bysubsequent pulses will show a certain amount of overlap. Thus, eachlocation of the surface of the substrate which is to be illuminated isgenerally provided with optical radiation from at least two pulses.Commonly, a relation between the pulse repetition frequency, a size of awindow of the substrate or other surface which is to be illuminated, anda scanning speed of the substrate or other to be illuminated object, ischosen such that each point of the surface of the substrate or other tobe illuminated surface is illuminated by a plurality of pulses. Due tophysical constraints, pulse energy of the pulsed source of radiation mayshow a certain amount of deviation. In other words, an energy providedby subsequent pulses may differ to a certain extent. Commonly, however,it is desirable to provide a homogeneous illumination, i.e., to arrangethat each point on the surface of the substrate or other to beilluminated object, is provided with a substantially same dose ofradiation. For this reason, a controller may be provided which drivesthe pulsed source of radiation thereby making use of a pulse energy ofthe source of radiation at previous pulses. To accomplish this, thecontroller may comprise a feed back loop incorporating an integrator. Bythe integrator, a total dose of pulses of radiation at a certainlocation may be stabilized to a certain extent by the integrative actionof the controller, which may easily be understood as subsequent pulsesilluminating a certain point at the surface of the substrate or otherobject will add to form a total dose at that point.

In more detail, a standard deviation SD of the integrated dose at anypoint of the wafer when the laser is controlled by the above describedcontroller may be expressed as:

${{SD}\mspace{11mu}({output})} = \frac{{SD}\mspace{11mu}({laser})}{N\left. \sqrt{}\;\left( {n/2} \right) \right.}$

wherein SD (output) represents a standard deviation of the output of thelaser when in the control loop, SD (laser) represents a standarddeviation of the pulsed source of radiation as is, thus without thecontroller, and N represents a number of laser pulses in a slit which isused to create a window of illumination onto the substrate or othersurface, and n represents a number of pulses in a slope of the slitprofile. Thus, it can be easily seen that a standard deviation of theoutput may be reduced by reducing the standard deviation of the laser aswell as by increasing the number of pulses in a slit, e.g., byincreasing a pulse repetition frequency or by decreasing a scanningspeed with which the substrate or other object is scanned.

The principle of controlling as described above has been used for a longtime in many applications. In fact, it is believed by the person skilledin the art that an improvement to the controller as described above maybe difficult.

SUMMARY

It is desirable to enhance a performance of the pulsed radiation sourcecontrol.

According to an embodiment of the invention there is provided alithographic apparatus comprising:

an illumination system configured to condition a radiation beam, theillumination system comprising a pulsed source of radiation and acontroller to control an output of the pulsed source of radiation, thecontroller comprising a dose sensor to measure a dose of a pulse of thesource of radiation, the dose sensor to provide a dose signalrepresentative of the measured dose, and an integrator unit to at leasttwice integrate the dose signal, an output of the integrator unit toprovide an integrator output signal comprising the at least twiceintegrated dose signal, the output of the integrator unit to drive adriving input of the source of radiation with the integrator outputsignal.

In another embodiment of the invention, there is provided anillumination system configured to condition a radiation beam, theillumination system comprising a pulsed source of radiation and acontroller to control an output of the pulsed source of radiation, thecontroller comprising a dose sensor to measure a dose of a pulse of thesource of radiation, the dose sensor to provide a dose signalrepresentative of the measured dose, and an integrator unit to at leasttwice integrate the dose signal, an output of the integrator unit toprovide an integrator output signal comprising the at least twiceintegrated dose signal, the output of the integrator unit to drive adriving input of the source of radiation with the integrator outputsignal.

According to a further embodiment of the invention, there is provided acontroller to control an output of a pulsed source of radiation, thecontroller comprising a dose sensor to measure a dose of a pulse of thesource of radiation, the dose sensor to provide a dose signalrepresentative of the measured dose, and an integrator unit to at leasttwice integrate the dose signal, an output of the integrator unit toprovide an integrator output signal comprising the at least twiceintegrated dose signal, the output of the integrator unit to drive adriving input of the source of radiation with the integrator outputsignal.

According to a still further embodiment of the invention, there isprovided a method to control an output of a pulsed source of radiation,the method comprising measuring a dose of a pulse of the source ofradiation, at least twice integrating the measured dose to provide anintegrator output signal comprising an at least twice integrated dosesignal, and drive a driving input of the source of radiation with theintegrator output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 a-2 b schematically depict a shape of a pulse window and anintensity profile of one or more pulses provided by the pulsed source ofradiation in accordance with an embodiment of the invention;

FIG. 3 schematically depicts a control diagram showing the pulsed sourceof radiation and the controller in accordance with an embodiment of theinvention; and

FIG. 4 a-4 e schematically depict details of (a part of) the controlleras depicted in FIG. 3.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B(e.g., UV radiation or any other suitable radiation), a mask supportstructure (e.g., a mask table) MT constructed to support a patterningdevice (e.g., a mask) MA and connected to a first positioning device PMconfigured to accurately position the patterning device in accordancewith certain parameters. The apparatus also includes a substrate table(e.g., a wafer table) WT or “substrate support” constructed to hold asubstrate (e.g., a resist-coated wafer) W and connected to a secondpositioning device PW configured to accurately position the substrate inaccordance with certain parameters. The apparatus further includes aprojection system (e.g., a refractive projection lens system) PSconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g., including one ormore dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The mask support structure supports, i.e., bears the weight of, thepatterning device. It holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The mask support structure can use mechanical, vacuum, electrostatic orother clamping techniques to hold the patterning device. The masksupport structure may be a frame or a table, for example, which may befixed or movable as required. The mask support structure may ensure thatthe patterning device is at a desired position, for example with respectto the projection system. Any use of the terms “reticle” or “mask”herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system.”

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g., employing a programmable mirror array of a typeas referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports” (and/or two or more masktables or “mask supports”). In such “multiple stage” machines theadditional tables or supports may be used in parallel, or preparatorysteps may be carried out on one or more tables or supports while one ormore other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques can beused to increase the numerical aperture of projection systems. The term“immersion” as used herein does not mean that a structure, such as asubstrate, must be submerged in liquid, but rather only means that aliquid is located between the projection system and the substrate duringexposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the mask support structure (e.g., mask table MT),and is patterned by the patterning device. Having traversed the mask MA,the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioning device PW and position sensor IF (e.g., aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g., so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1) can be used to accuratelyposition the mask MA with respect to the path of the radiation beam B,e.g., after mechanical retrieval from a mask library, or during a scan.In general, movement of the mask table MT may be realized with the aidof a long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the first positioning device PM.Similarly, movement of the substrate table WT or “substrate support” maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT or “mask support” and the substratetable WT or “substrate support” are kept essentially stationary, whilean entire pattern imparted to the radiation beam is projected onto atarget portion C at one time (i.e., a single static exposure). Thesubstrate table WT or “substrate support” is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT or “mask support” and the substratetable WT or “substrate support” are scanned synchronously while apattern imparted to the radiation beam is projected onto a targetportion C (i.e., a single dynamic exposure). The velocity and directionof the substrate table WT or “substrate support” relative to the masktable MT or “mask support” may be determined by the (de-)magnificationand image reversal characteristics of the projection system PS. In scanmode, the maximum size of the exposure field limits the width (in thenon-scanning direction) of the target portion in a single dynamicexposure, whereas the length of the scanning motion determines theheight (in the scanning direction) of the target portion.

3. In another mode, the mask table MT or “mask support” is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT or “substrate support” is moved or scanned while apattern imparted to the radiation beam is projected onto a targetportion C. In this mode, generally a pulsed radiation source is employedand the programmable patterning device is updated as required after eachmovement of the substrate table WT or “substrate support” or in betweensuccessive radiation pulses during a scan. This mode of operation can bereadily applied to maskless lithography that utilizes programmablepatterning device, such as a programmable mirror array of a type asreferred to above.

Combinations and/or variations on the above-described modes of use orentirely different modes of use may also be employed.

FIG. 2 a schematically depicts a top view of a part of a surface of asubstrate W or other surface of an object which is to be illuminated,and shows a shape of consecutive pulses indicated by pulse window PW1,pulse window PW2 and pulse window PW3. A direction of scanning isindicated by arrow ARR. The scanning may be performed by moving thesubstrate W or other object with respect to the pulsed source ofradiation, by a suitable manipulation in an optical protection systemsuch as the projection system PS indicated in FIG. 1, or by, forexample, moving the pulsed source of radiation with respect to thesubstrate W. As scanning is performed in the direction indicated byarrow ARR, a first pulse illuminates, for example, window PW1, a secondpulse illuminates window PW2, a third pulse illuminates window PW3. Asshown in FIG. 2 a, the window PW1, PW2, PW3 show a certain amount ofoverlap. In a practical implementation, more or less overlap may bepresent, it is, e.g., imaginable that in embodiments of a lithographicapparatus that tens of pulses overlap each other, e.g., 20, 40, 60 or 80pulses. In the schematic representation shown in FIG. 2 a, an overlap ofthree pulses is depicted.

Although in FIG. 2 a the pulse windows PW1, PW2, PW3 are shown to have arectangular shape, any shape may be possible, such as a circular, oval,triangular, square or any other shape. An intensity of subsequent pulseson a location on the substrate is depicted in FIG. 2 b. FIG. 2 b showsalong a horizontal axis a time, i.e., subsequent pulses are depicted. Avertical axis in the graph in FIG. 2 b shows an intensity of thesubsequent pulses as measured on a single location on the substrate. Itis remarked that the example as shown in FIG. 2 b comprises asubstantially larger overlap as compared to the example according toFIG. 2 a. In other words, a single location on the substrate isilluminated by a larger number of pulses during the scanning. Asdepicted in FIG. 2 b, in a mid-region, the pulses show a relativelyuniform amount of energy (an amount of pulses in this region commonlybeing indicated as N), while near the edges Edg, a slope may be observed(an amount of pulses in this region commonly being indicated as n). Itis noted that the number N may be understood as comprising the number ofpulses at a location which have an intensity at that location of 50% ormore of the intensity of pulses in the mid region at that location.Thus, at the edges EDG, an intensity of the pulse gradually changes fromits value in the mid-region towards zero. Commonly, the pulse windowPW1, PW2, PW3 may have been formed by a slit which may be positioned,e.g., in the patterning device such as the mask MA. Due to, e.g.,physical effects such as diffraction, the slope may come into existence.

Referring to FIG. 1, the pulsed source of radiation may as an example becomprised in the source SO. Further detail of the illumination systemwill now be described with reference to FIGS. 3 and 4 a-e.

FIG. 3 shows a pulsed source of radiation in this example a pulsed laserLSR. The pulsed source of radiation may however also comprise any othertype of radiation source, such as, for example, a light emitting diode(LED), a diode laser, a laser including a conversion system, such as aRAMAN conversion, a gas discharge lamp, a filament lamp. The laser mayfurther comprise any type of laser, such as, e.g., a solid state laser.The term pulsed source of radiation may thus be understood as any typeof source which provides optical energy in a form of pulses, thusintermittently providing optical energy. The pulsed source of radiation,such as the laser LSR is controlled by a controller which in thisexample comprises a dose-sensor SENS which measures a dose of a pulsePLS of the source of radiation LSR. The dose sensor may comprise anytype of optical sensor, such as, for example, but not limited to a photodiode, a photo resistive element, a photo conductive element. Ingeneral, the dose sensor may comprise any measurement device ormeasurement component which is configured to convert optical energy intoa signal, such as, for example, an electrical signal, optical signal.The signal indicated here as a dose signal, which is thus representativeof the measured dose, is provided by the dose sensor SENS at the dosesensor output DSO which may thus comprise, for example, an electricaloutput, optical output.

The controller further comprises an integrator unit I, an integratorinput II of the integrator unit I being connected to the dose sensoroutput DSO. The integrator unit, which will be discussed in more detailbelow thus is able to integrate the dose signal provided by the dosesensor SENS at the dose sensor output DSO. An integrator output IO ofthe integrator unit I is provided to a driving input DRV of the sourceof radiation, thus in this example the laser LSR. In this example, a setpoint SETP is further provided, the set point being combined with theintegrator output signal provided at the integrator output IO, beforebeing provided to the driving input DRV of the source of radiation. Inthis example, the integrator output signal is subtracted from the setpoint, thus a difference between the setpoint SETP and the integratoroutput signal at the integrator output IO being provided to the drivinginput DRV of the source of radiation. The effect that the integratoroutput signal is in this example subtracted from the set point signal isbecause it is desirable to obtain a negative feed back in the controlloop formed by the pulsed source of radiation (in this example the laserLSR) and the controller (in example comprising the dose sensor SENS andthe integrator I). Instead of the subtraction to obtain a negative feedback other possibilities are that the integrator could, for example,have a negative gain. For schematic and explanatory purposes, the minussign has however been incorporated in the combination between the setpoint and the integrator output.

As an alternative to the set point SETP shown in FIG. 3, it will beappreciated that the pulsed source of radiation may comprise a separateset point input to which the set point is connected and a separatemodulation input which may serve as the driving input to which theintegrator output signal may be connected. In accordance with anembodiment of the invention, the integrator unit integrates the dosesignal at least twice, thus comprising, e.g., a double integrator. Theintegrator output IO thus provides an integrator output signalcomprising an at least twice integrated dose signal. The inventors havedevised that by providing that the integrator unit integrates the dosesignal at least twice, a lower standard deviation of the output pulsePLS may be provided as compared to a conventional control system for thepulsed source of radiation which comprises a single integrator, thus anintegrator unit comprising a single integrator. In particular, theinventors have devised that the standard deviation of the integrateddose at any point of the substrate when the pulsed source of radiation(such as the laser) is controlled by above described controller, may beexpressed as:

${{SD}\mspace{11mu}({output})} = \frac{{SD}\mspace{11mu}({laser})}{N\mspace{11mu}\left( {n/2} \right)}$

An explanation of the terms used in the formula having been providedabove. One of the benefits of the controller according to thisembodiment of the invention is that it lowers the standard deviation ofthe output of the laser pulses, thus providing for a more constantillumination of the substrate W or other object to be illuminated, at agiven standard deviation of pulses of the laser LSR itself, a givennumber of pulses N in the slit and a number of pulses n in the slope atthe edge Edg of the slit. A further benefit may be that a same amount ofuniformity of illumination of the surface of the substrate or otherobject may be obtained with a lower pulse repetition rate of the pulsedsource of radiation and/or with a higher scanning speed of the scanningof the substrate or other object. Thus, embodiments of the inventionmay, e.g., allow to enhance uniformity of illumination, to increase ascanning speed, thus increasing a throughput of the lithographicapparatus or other equipment in which the pulsed source of radiation andthe controller is comprised, and/or to reduce cost, as given a certainamount of uniformity in illumination, a lower pulse repetition rate maybe applied, thus making usage of a lower cost source of radiationpossible (e.g., a lower cost laser having a lower pulse repetitionfrequency).

The controller, and in particular the integrator unit, may beimplemented using analogue electronics, comprising, e.g., analogueintegrators, multipliers, amplifiers, adders. However, it will beappreciated that the integrator unit may comprise digital electronicssuch as programmable integrated circuits, microprocessors,microcontrollers. The integrator unit may consist of hardware. However,it will be appreciated that the integrator unit at least in part may beimplemented using software instructions to be executed by amicroprocessor, microcontroller or other programmable device. In anembodiment of the invention, combinations of analogue and/or digitalhardware and software are equally well possible. Further, as explainedabove, the dose sensor SENS may comprise any type of optical sensor. Inaddition, the dose sensor may also comprise suitable read outelectronics, or other read out devices, such as, for example, a read outamplifier, a buffer, a sample and hold device, a pulse memory whichstores a value representative of a pulse energy of a pulse. Also, theadding or subtracting function where the set point is added to theintegrator output IO, as illustrated in FIG. 3, may be implemented usinganalogue and/or digital hardware, and may also be implemented at leastin part in software.

FIG. 4 a-4 d each depict a schematic view of an embodiment of theintegrator unit I as depicted and described with reference to FIG. 3.The integrator input II and integrator output IO as identified in FIG.3, are also indicated in FIGS. 4 a-4 d. Thus, FIGS. 4 a-4 d each providean example of a built up of the integrator unit I as depicted in FIG. 3.In addition to the elements of the exemplary embodiments as depicted anddescribed with reference to FIGS. 4 a-4 d these embodiments may comprisefurther elements, structures such as, for example, amplifiers, adders,multipliers, integrators, dividers.

The embodiment of the integrator unit according to FIG. 4 a comprises afirst integrator stage I1 and a second integrator stage I2. A firstamplifier GI1 is connected in series with the first integrator stage anda second amplifier GI2 is connected in series with the second integratorstage I2. Further, a bypass amplifier GP is provided which bypasses thesecond integrator stage I2 and the second amplifier stage GI2. An outputsignal of the bypass amplifier GP is added by an adder ADD to an outputof the series connection of the second integrator stage I2 and thesecond amplifier stage GI2. The integrator stages I1 and I2 may eachcomprise a single integrator, thus each providing an integration. Theintegrator output signal IO in this example comprising a combination ofa twice integrated dose signal as provided to the integrator input II,the twice integrated signal being provided by the series connection ofthe first and second integrator stages I1, I2 and the first and secondamplifiers GI1, GI2. Further, the integrator output signal as providedat the integrator output IO may comprise a once integrated dose signalwhich is provided via a path comprising the bypass amplifier GP, thefirst integrator I1 and the first amplifier GI1. The amplifiers GI1, GI2and GP may have any amplification factor. In an embodiment, theamplification may also be set to 0, and in that case the respectiveamplifier may be omitted. By selecting a gain for each of the amplifiersGI1, GI2 and GP, a mixture of once and twice integrated dose signals maybe provided at the integrator output: when, for example, GP is set tozero, then the integrator output signal comprises a twice integrateddose signal only. In this example, GI1 and GI2 can be set to any, nonzero value, a gain of these amplifiers influencing a loop gain of acontrol loop formed by the controller and the pulsed source of radiationas depicted in FIG. 3. When GI2 is set to zero, then only the path viaGP and GI1 provides for an integrator output signal, the integratoroutput signal in that example comprising a once integrated dose signal.

The implementation as shown in FIG. 4 a as well as the implementationsas shown in FIGS. 4 b and 4 d provide the benefit that by choosingappropriate gain factors for the respective amplifiers, the outputsignal provided at the integrator output may comprise the onceintegrated dose signal, the twice integrated dose signal and/or anycombination thereof, which thus provide flexibility in the dimensioningof the controller to cope with specific requirements of the applicationin which the illumination system and/or controller are incorporated, thecharacteristics of the pulsed source of radiation (such as the laser,etc.) To further enhance flexibility, the amplifiers, or at least one ofthe amplifiers GI1, GI2, GP may comprise a programmable amplificationfactor. By the programmable amplification factor, a desired value forthe amplification may be programmed into the respective amplifier, e.g.,by a computer or microprocessor. To accomplish this, the amplifiers maybe implemented fully or in part in software, which may enable to alteran amplification factor in a numeric way. Also, in an embodiment, theamplifiers comprise a plurality of amplification factors, e.g., using aresistive network comprising, e.g., a ladder network of resistors, andby selecting an appropriate one or appropriate ones of the resistors,e.g., by switches which may be operated by a control device, by amicrocontroller, microprocessor. Also, the programmable amplifiers maycomprise variable resistors (comprising, e.g., a field effecttransistor, a potentio meter), the variable resistor being driven, forexample, by a control device, microprocessor, microcontroller. Abeneficial selection for the amplification factors may be obtained whenthe bypass amplifier GP comprises a gain of substantially 1, the secondamplifier GI2 comprises a gain of substantially 1 and the firstamplifier GI1 comprises a gain of substantially 0.5. This provides anadequate performance when a large number of pulses is in a slit, thuswhen a high amount of overlap between successive windows projected ontothe substrate or other surface, is present. In particular, performancemay be beneficial compared to a conventional controller when the numberof pulses in the slope is larger than 2 (i.e., n>2), which may ingeneral be accomplished when the number of pulses N in the slit islarger than 30. With a higher number of pulses, in particular with twiceor more these values, the performance of the controller may show asignificant improvement as compared to a conventional controller. Bysetting the amplification factor of the bypass amplifier GP to 1, thefirst amplifier GI1 to 1 and the second amplifier GI2 to 0, a singleconventional integrator may be emulated. Thus, the embodiment as shownin FIG. 4 a also makes it possible to revert back to the controllerwhich has a single integrator by appropriate setting of the gainfactors. In general, a beneficial range for the amplification factors ofthe amplifiers GI1, GI2, GP is in a range between 0 and 1.

FIG. 4 b shows an alternative embodiment of the integrator unit I. Thedose signal provided at the integrator input II is first integrated bythe first integrator stage II, an output of the first integrator stageI1 being provided to a second integrator stage 12 connected in serieswith an amplifier, in this example a second amplifier GI2. An amplifierGI1 may be connected in series with the first integrator stage I1, asshown in the embodiment of FIG. 4 b. The second integrator stage I2 andthe second amplifier GI2 are bypassed by bypass amplifier GP, thus theoutput of the first integrator stage I1 being also provided to an inputof the amplifier GP. The output of the series connection of the secondintegrator stage I2 and the second amplifier GI2 is added to the outputof the bypass amplifier GP by an adder ADD to provide the integratoroutput signal IO. With the exemplary embodiment as depicted in FIG. 4 b,the integrator output signal at the integrator output IO may comprise aonce integrated dose signal provided via the first integrator stage I2and the amplifiers GI1, GP, and/or a twice integrated dose signalprovided by the first and second integrator stages I1, I2 and the firstand second amplifiers GI1, GI2. By choosing appropriate values for theamplification factors of the first and second amplifiers GI1, GI2 andthe bypass amplifier, any combination of the once and twice integrateddose signal may be provided at the integrator output. Further, benefitsas discussed with respect to the embodiment shown in FIG. 4 a are,mutatis mutandis, also applicable to the embodiment shown and describedwith reference to FIG. 4 b. The inventors have devised that a beneficialoperation is obtained when selecting the am amplification of the bypassamplifier and of the second amplifier GI2 such as to be substantially 1,while an amplification of the first amplifier GI1 may be selected to besubstantially 0.5.

FIG. 4 c shows an embodiment comprising three integrator stages I1, I2,I3. The integrator stages I1, I2, I3 are in this example connected inseries. An output signal at the integrator output IO thus comprises atriple integrator dose signal as provided to the integrator input II.The embodiment as shown in FIG. 4 c may further comprise one or moreamplifiers (e.g., connected in series with the series connection ofintegrators), one or more bypass amplifiers which bypass one or more ofthe integrator stages I1, I2, I3. Such controller has beneficialperformance when, e.g., the profile as depicted and described withreference to FIG. 2 b includes higher order profiles (e.g., paraboliccurve-parts, including, e.g., 2nd order profiles) in addition to forexample the trapezoid shape that consists of lower order (e.g., 0th and1st order) profiles only.

FIG. 4 d shows a still further embodiment of the integrator unit I. Theembodiment shown in FIG. 4 d comprises a parallel connection of twobranches. A first branch comprises a first integrator stage I1 and afirst amplifier GI1, a second branch comprising a second integratorstage I2 and the third integrator I3 and a second amplifier GI2. Boththe first and second branch is provided with the dose signal provided atthe integrator input. Outputs of the branches are added by the adderADD. The first branch provides by the first integrator stage I1 and thefirst amplifier GI1, a once integrated dose signal to the integratoroutput IO. The second branch provides via the second integrator stage I2and the third integrator stage I3 as well as the second amplifier GI2 atwice-integrated dose signal to the integrator output IO. As explainedabove with reference to FIGS. 4 a and 4 b, by choosing appropriateamplification factors for the first and second amplifiers GI1, GI2, adesired combination of a once integrated dose signal and a twiceintegrated dose signal may be provided at the integrator output IO. Dueto the slightly different architecture of the embodiment shown in FIGS.4 b and 4 d, amplification factor might differ somewhat from theamplification factors as described with reference to FIG. 4 a.

FIG. 4 e schematically depicts an implementation of an integrator stage,in this example the integrator I3 as depicted in FIGS. 4 a-4 d mayhowever be identical. The embodiment as shown in FIG. 4 e comprises adelay, here indicated as z⁻¹, the delay being substantially equal to atime between successive pulses of the pulsed source of radiation. Anoutput of the delay z⁻¹ is provided to an output of the integrator stageand fed back to an input thereof. At the input, an input signal to theintegrator stage is added by an adder ADD to the output signal of theunit delay z⁻¹ and provided as input to the unit delay. In particular,in a digital or numeric implementation, the integrator of FIG. 4 e maybe implemented, e.g., in software or dedicated hardware, thus makinganalogue implementations (requiring, e.g., large capacitors, highprecision, low leakage electronics, etc.) superfluous.

In addition to the elements of the integrator unit has depicted in theexemplary embodiments according to FIGS. 4 a-4 d, the controller mayfurther comprise an amplifier having a amplification factor which may besubstantially an inverse of or less than an inverse of an amplificationfactor of the source of radiation in case that the dose sensor performsa (substantially) calibrated measurement. Thereby, an optimum loopamplification factor in a control loop consisting of the pulse source ofradiation LSR, the dose sensor sense, and the integrator I as depictedin FIG. 3, may be obtained. The amplifier may be connected in serieswith the embodiments shown in FIGS. 4 a-4 d thus, e.g., directly at theintegrator input II or the integrator output IO. To obtain a highstability in the control loop, it is preferred that the amplificationfactor of the amplifier is less than the inverse of the amplificationfactor of the source of radiation.

As in a further embodiment of the invention, it is possible in any ofthe embodiments of the integrator unit, such as the embodiments depictedand described with reference to FIGS. 4 a-4 d, that at least one of theintegrator stages comprises a leaking integrator stage, to prevent awind up or saturation in case of a, e.g., an input to the integratorhaving a high value or a input which lasts for more than average time,thus leading to a high value at the output of the integrator stages(s).The controller as described here may be used in an illumination systemof a lithographic apparatus, however may have numerous otherembodiments. In an embodiment, the controller may be used in anyillumination system which comprises a pulsed source of radiation.

According to an embodiment of the invention, there is provided a methodincluding measuring a dose of a pulse PLS of the source of radiationLSR, at least twice integrating the measured dose to provide anintegrated output signal comprising an at least twice integrated dosesignal, and driving a driving input DRV of the source of radiation LSRwith the integrated output signal.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion,” respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens,” where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practised otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g., semiconductor memory, magnetic or optical disk) havingsuch a computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus comprising: an illumination systemconfigured to condition a radiation beam, the illumination systemcomprising a pulsed source of radiation and a controller to control anoutput of the pulsed source of radiation, the controller comprising: (a)a dose sensor configured to measure a dose of a pulse of the pulsedsource of radiation and to provide a dose signal representative of themeasured dose; and (b) an integrator unit configured to at least twiceintegrate the dose signal, an output of the integrator unit to providean integrator output signal comprising the at least twice integrateddose signal, the output of the integrator unit to drive a driving inputof the source of radiation with the integrator output signal.
 2. Thelithographic apparatus according to claim 1, wherein the integratoroutput signal further comprises a once integrated dose signal.
 3. Thelithographic apparatus according to claim 2, wherein the integrator unitcomprises a first integrator stage to integrate the dose signal and asecond integrator stage to provide the twice integrated dose signal, andamplifiers to amplify the once and twice integrated dose signals.
 4. Thelithographic apparatus according to claim 3, wherein the amplifierscomprise a programmable amplification factor.
 5. The lithographicapparatus according to claim 3, wherein the integrator unit comprises aseries connection of the two integrator stages and two of theamplifiers, the integrator unit further comprising a bypass amplifier tobypass a first of the integrator stages and a first of the amplifiers.6. The lithographic apparatus according to claim 5, wherein the bypassamplifier comprises an amplification factor of substantially 1, whereinthe bypassed of the two of the amplifiers in the series connectioncomprises an amplification factor of substantially 1 and wherein theother of the two of the amplifiers in the series connection comprises anamplification factor of substantially 0.5.
 7. The lithographic apparatusaccording to claim 3, wherein the integrator unit comprises a seriesconnection of the two integrator stages, a first of the amplifiers toamplify a once integrated dose signal from an output of the first of theintegrator stages, and a second of the amplifiers to amplify the twiceintegrated dose signal from an output of the second of the integratorstages.
 8. The lithographic apparatus according to claim 1, wherein thecontroller further comprises an amplifier having an amplification factorwhich is substantially an inverse of an amplification factor of thesource of radiation.
 9. The lithographic apparatus according to claim 1,wherein the controller further comprises an amplifier having anamplification factor which is substantially less than an inverse of anamplification factor of the source of radiation.
 10. The lithographicapparatus according to claim 1, wherein the integrator unit comprises atleast one leaking integrator stage.
 11. The lithographic apparatusaccording to claim 1, wherein the integrator unit comprises first andsecond integrator stages configured to twice integrate the dose signaland further comprises a third integrator stage, the third integratorstage configured to integrate the twice integrated dose signal toprovide a triple integrated dose signal, the integrator output signal atthe integrator output to comprise the triple integrated dose signal. 12.An illumination system configured to condition a radiation beam, theillumination system comprising a pulsed source of radiation and acontroller to control an output of the pulsed source of radiation, thecontroller comprising: a dose sensor configured to measure a dose of apulse of the pulsed source of radiation and to provide a dose signalrepresentative of the measured dose; and an integrator unit configuredto at least twice integrate the dose signal, an output of the integratorunit to provide an integrator output signal comprising the at leasttwice integrated dose signal, the output of the integrator unit to drivea driving input of the source of radiation with the integrator outputsignal.
 13. A controller to control an output of a pulsed source ofradiation, the controller comprising: a dose sensor configured tomeasure a dose of a pulse of the pulsed source of radiation and toprovide a dose signal representative of the measured dose; and anintegrator unit configured to at least twice integrate the dose signal,an output of the integrator unit to provide an integrator output signalcomprising the at least twice integrated dose signal, the output of theintegrator unit to drive a driving input of the source of radiation withthe integrator output signal.
 14. A method of controlling an output of apulsed source of radiation, the method comprising: measuring a dose of apulse of the pulsed source of radiation; at least twice integrating themeasured dose to provide an integrator output signal comprising an atleast twice integrated dose signal; and controlling the pulsed source ofradiation with the integrator output signal.
 15. A lithographicapparatus comprising: a pulsed source of radiation configured to outputa beam of radiation; a controller configured to control an output of thepulsed source of radiation, the controller comprising: (a) a dose sensorconfigured to measure a dose of a pulse of the pulsed source ofradiation and to provide a dose signal representative of the measureddose, and (b) an integrator unit configured to at least twice integratethe dose signal, an output of the integrator unit to provide anintegrator output signal comprising the at least twice integrated dosesignal, the output of the integrator unit to drive a driving input ofthe source of radiation with the integrator output signal; anillumination system configured to condition the beam of radiation; asupport configured to support a patterning device, the patterning deviceconfigured to pattern the beam of radiation to provide a patterned beamof radiation, and a projection system configured to project thepatterned beam of radiation onto a surface of a substrate.